The high aerobic glycolysis in a variety of tumor cells has been shown to be caused by an abnormal increase of intracellular ATPase activity. In the case of Ehrlich ascites tumor cells and neuroblastoma cells grown in tissue culture the activated ATPase is in the plasma membrane (Na ion K ion ATPase). The increased ATPase activity is shown to be caused by a decreased efficiency of ion pumping and can be controlled by certain flavonoids. Addition of these flavonoids inhibit glycolysis of intact cells. In some other cells (e.g. polyoma virus transformed 3T3 cells) the activated ATPase is located in the mitochondria. Again it is not an impairment of phosphorylation function but a matter of efficiency. Flavonoids also improve the efficiency of mitochondrial phosphorylation by inhibiting the ATPase activity. Experiments with cells grown in tissue culture show that certain bioflavonoids inhibit growth at low concentrations provided the concentration of serum in the growth medium is low and bicarbonate is substituted by an organic buffer.